Photosynthesis
Objectives
• Following this section, students will:
– Understand importance of photosynthesis in plant
growth
– Be able to write the balanced equation for
photosynthesis
– Understand key components of light and dark
reactions
– Be able to describe how photosynthesis is
measured
– Understand the influence of environmental factors
on photosynthesis
Initial plant weight = 1 lb
Initial soil weight =100 lbs
After 10 years...
Plant weight = 51 lbs
Soil weight = ?
a. 50 lbs
b. 100 lbs
c. between 75 - 85 lbs
d. slightly less than 100 lbs
>95% of plant dry
weight biomass comes
from photosynthesis
Why study photosynthesis?
• Ultimate basis of energy in all biological
systems
• All food and fiber produced on earth is
ultimately due to photosynthesis
• Increasing interest in plants as sinks for
carbon
Why study photosynthesis?
• All organic compounds needed for plant
growth (proteins, lipids, starches,
sugars) are derived from the products of
photosynthesis
• Therefore, any abiotic or biotic factor
that limits photosynthesis will ultimately
limit plant growth and survival
Photosynthesis:
balanced reaction
Light
6CO2 + 6H2O --------------> C6H12O6 + 6O2
Carbon
Water
Glucose Oxygen
dioxide
Photosynthesis
• Two reactions
• Light
– Light dependent
• Dark
– Light independent
Light reaction
•
•
•
•
Light energy absorbed by chlorophyll
Traps energy in form of ATP
Splitting of water (releases O2)
Additional energy stored in NADPH
Dark reactions
(Calvin cycle)
• Cyclic process
• Uses energy stored as ATP and
NADPH in light reactions
• CO2 molecule combines with 5-carbon
sugar to form two 3-carbon molecules
(C-3 photosynthesis)
• 3-carbon molecules are joined to form
6- carbon sugar (glucose)
Chloroplasts
•
•
•
•
Site of photosynthetic reactions
Chloroplasts are a membrane system
Contain chlorophyll
Made up of:
– Thylakoid membranes - stacked into grana
– Stroma
Plant Pigments
• Chlorophylls
• Carotenoids
• Carotenes
• Xanthophylls
• Anthocyanins
Photosynthetic pigments
Chlorophyll action spectrum
Light Quantity
• Instantaneous
• Cumulative – Daily Light Integral
Effect of light quantity:
Photosynthetic light response curve
Photosynthetic Light Response Curves
Photosynthetic light response curves
• Dark respiration rate (Daytime respiration rate)
– Net photosynthetic (Pn) rate when irradiance is 0
• Light compensation point
– Irradiance level at which Pn = 0
• Light saturation point
– Irradiance level where Pn is maximal
• Quantum yield
– Slope of the linear phase of the curve
– moles of CO2 fixed per mole photons absorbed
Leaf-level vs. higher level Pn
measurements
Photosynthetic Light Response Curves
•
•
Genetic vs. plastic differences.
The shape of the curve is determined
by three variables:
1. Leaf area per unit mass – an increase
provides greater surface area for PAR
absorption.
2. Respiration and photorespiration: a
decrease lowers the compensation point.
3. The capacity of the photosynthetic
system
– Primarily RUBISCO levels
Genetic vs. Plastic Differences
Leaf Area
Leaf Morphology
Base of Plant
Top of Plant
Representative values of Amax
Species
Populus deltoides
Populus trichocarpa
Hybrid (t x d)
Amax (mol m-2 s-1)
8.0-15.9
10.0-22.0
18.0-23.0
Pinus taeda
Pinus sylvestris
Pinus contorta
5.0-6.3
3.0-6.7
2.0-3.2
Dark reactions
(Calvin cycle)
• Uses energy stored as ATP and
NADPH in light reactions
• CO2 molecule combines with 5-carbon
sugar to form two 3-carbon molecules
(C-3 photosynthesis)
• 3-carbon molecules are joined to form
6- carbon sugar (glucose)
RUBP carboxylase (RUBISCO) is
a key enzyme is photosynthesis
• Most abundant protein on earth
• Also acts as an oxygenase
• Photorespiration (uses O2)
Calvin cycle summary
• 5 carbon compound (RUBP) combines
with CO2 to produce 3-C compound (3PGA)
• Uses energy from light reactions in the
form of ATP and NADPH
• Key enzyme is RUBP-carboxylase
How do we measure photosynthesis?
What determines
photosynthetic productivity?
• Environmental factors
• Internal control
What determines
photosynthetic productivity?
• Environmental factors
• Internal control
Environmental factors that limit
photosynthesis
•
•
•
•
•
Light
Water
Temperature
Nutrition
CO2
Net photosynthesis
(µmol CO2/m2/s)
• Supplemental lighting is most effective when the sunlight level
is low. This is because of the “diminishing returns” shape of
the response curve. (from Lighting Up Profits)
2. Photosynthesis
2.2 Environmental factors: Carbon dioxide
Net photosynthesis
(µmol CO2/m2/s)
• As CO2 increases to ≈ 900 ppm, photosynthesis rate
increases. (from Lighting Up Profits)
2. Photosynthesis
2.2 Environmental factors: Temperature
Net photosynthesis
(µmol CO2/m2/s)
• Metabolic processes, including photosynthesis, increase to
some optimum temperature.
• At low temperature, enzymes are not active.
• At high temperature, plant damage may occur.
How efficient is the conversion of
light energy to plant tissue?
• Four major factors:
– PPF absorption efficiency
– Photosynthetic efficiency
– Respiration efficiency
– Harvest index
Daily Light Integral and Crop Quality
• DLI influences:
•
•
•
•
•
•
•
Flower induction
Flowering percentage
Flower number
Flower size
Branching
Plant height
Growth – shoot and root
DLI and Flower Induction
Hibiscus cisplatinus flowers earlier as total daily light increases
Digitalis purpurea ‘Foxy’
Flowering Percentage
100
80
60
40
20
0
0
5
10
15
20
25
-2 -1
Average Daily Light Integral (mol m d )
Diascia ‘Flying Colors Coral’
68 ºF
5.3 mol m-2 d-1
15.5 mol m-2 d-1
Nemesia ‘Sunsatia Peach’
Days to Flower
15.5 mol m-2 d-1
41
38
32
47
DNF
58
5.3 mol m-2 d-1
45
44
DNF
57
62
68
73
DNF
79 ºF
Salvia farinacea ‘Strata’
Lateral Inflorescences
15.5 mol m-2 d-1
5.9
8.6
8.6
8.4
7.0
5.3 mol m-2 d-1
DNF
57
4.1
4.5
62
68
4.6
73
6.1
79 ºF
Tagetes patula ‘Bonanza Yellow’
Daily Light Integral (mol·m-2·d-1):
24
20
57 oF
68 oF
79 oF
= 1 inch
10
Daily Light Integral
1 GJ = 1000 MJ
1 MJ = 2 mol
1.4 g MJ-1= 2.8 g mol-1
DLI (mol·m-2·d-1):
Celosia
Marigold
4
7
14
What determines
photosynthetic productivity?
• Environmental factors
• Internal control
Source-sink relations
• Source
– Point where carbohydrates are produced
(leaves) or released from storage (roots,
stems)
• Sink
– point on the plant where carbohydrates are
used for growth or stored
Source-sink relations
• Some organs may alternate between
being a sink or a source
• Examples:
– newly emerging leaves are a sinks then
become sources
– roots may be sources in early spring then
becomes sinks in fall
Source-Sink strength
1. Rapid leaf growth
Source-Sink strength
2. Kernal filling
Source-Sink strength
3. Kernals full: basipetal transport only
Phloem
• Carbohydrates more throughout the
plant via the phloem
• Made up of sieve tube elements and
companion cells
• Process by which carbohydrates move
into phloem is called ‘phloem loading’
Feedback inhibition
• Regulation of photosynthesis by sink
• Unloading of phloem at sink is can’t
keep up with assimilation at source
• Pipes “back-up”
– Evidence:
– Fruit thinning
– Girdling
C-4 leaf anatomy
Summary of C3, C4, & CAM
C3
C4
CAM
Enzyme
Rubisco
PEPCase
PEPCase
Opt . Temp
15-25oC
30-40oC
35oC
Habit at
Relat ively
moist
22
Arid or
t ropical
39
Arid
Product ivit y
(T/ha/y)
Low & variable
Alternate photosynthetic
pathways
• CAM
• C4 photosynthesis